How do I examine this Echelon form Matrix further?

The matrix which I have is a 3x5 matrix. The first row has values [1 2 1 2 1], 2nd [1 2 0 1 1] and 3rd [1 2 1 2 2]. I have turned it into echelon form and found that it is as follows: 1st [1 2 0 1 1], 2nd [0 0 1 1 0] and 3rd [0 0 0 0 1]. Now the question that I am asked is to confirm this result by examination of third order determinants. How do I do this and if possible, provide an answer . Thanks